1. Field of the Invention
The present invention relates to a method for manufacturing bulky nonwoven fabrics. More specifically, this invention is concerned with a method for manufacturing bulky nonwoven fabrics suitable for surfacings for disposable diapers, interlinings or waddings for clothing and so on.
2. Statement of the Prior Art
In recent years, there have been developed spun bonding techniques for making nonwoven fabrics by drawing and stretching a fiber bundle comprising a plurality of single fibers spun out of a spinneret with a high-speed stream of air and depositing it on a net conveyor to form a web, followed by heat treatments, some of which have been carried out on an industrial scale. According to one of the spun bonding techniques known for obtaining bulky nonwoven fabrics (Japanese Patent Application Laid-Open No. 1471/1973), for example, latently crimpable composite fibers are deposited on a net conveyor to form a web, which is then heat-treated into a crimped nonwoven fabric. According to another spun bonding technique (Japanese Patent Application Laid-Open No. 282350/1988), apparently crimping composite fibers are formed into a fiber bundle, which is then deposited on a net conveyor to form a web to be processed into an nonwoven fabric.
When a nonwoven fabric having little unevenness of weight per unit area referring as unit-weight hereafter is to be obtained by such spun bonding techniques, it is then required to prevent the once deposited web from being disturbed or blown off by a high-speed stream of air reflected off the upper face of the net conveyor. To this end, an exhaust device is provided back side of a region of the net conveyor against which the web is to be blown, thereby passing and sucking a substantially whole portion of the air stream through the net conveyor.
According to the conventional spun bonding techniques, the web is pressed against the net conveyor by the force of a high-speed air stream blown against it and the suction force of the exhaust device, as mentioned above. In consequence, the web is delivered to the next heat treatment step while it remains firmly engaged with the net conveyor. Accordingly, when a bulky nonwoven fabric is to be made with latenly crimpable fibers, any free development of crimps is inhibited even though the fibers are heat-treated so as to develop crimps, since they are firmly engaged and entangled in the net. This tends to offer such disadvantages as insufficient bulking, occurrence of unevenness of shrinkage and density on the web, and transfer of the network pattern of the net conveyor to the back side of the nonwoven fabric. Such disadvantages are liable to occur particularly in manufacturing bulky nonwoven fabrics of low unit-weight, which are used as the surfacings for disposable diapers, etc., and moreover have an influence upon commercial value. It is thus strongly desired to eliminate such disadvantages.